In Non-patent Document 1, a method of extracting a feature value in a pattern recognition (pattern discrimination) for discriminating a human and others from an image is disclosed. More specifically, the image is divided into plural areas in a reticular pattern and a histogram in the direction of a brightness gradient calculated for each of the areas is employed as a feature value.
In Patent Document 1 and Non-Patent Document 2, methods of extracting a feature value for classifying the types of the texture images are described. A technology described in Patent Document 1 employs a simultaneous occurrence matrix having elements P(i, j), which are the numbers of combinations in which a concentration of a point shifted in parallel from a point having a concentration i in certain distance and direction is j in an image as the feature value. In the texture image, since similar patterns appear repeatedly equidistantly, the feature value representing the simultaneous occurrence of concentration values at two points apart from each other at an equal distance is effective for discrimination of the texture. A technology in Non-Patent document 2 extracts a robust feature value with respect to an illumination variation by using the simultaneous occurrence matrix in the direction of the brightness gradient instead of the concentration value of the image.